1. Field of the Invention
The present invention relates to a manufacturing process for converting coal ash slag into a marketable cement product having the characteristics and qualities of portland cement. This process comprises the steps of transferring molten slag from a slagging coal gasifier to a melt chamber, reacting a mineral containing lime, for example: lime, quicklime or limestone, with said slag in the ratio of one part of slag to 1.2 to 4 parts of the mineral containing lime to form a homogeneous cement product, and transferring the cement product to a quench chamber where it cools and solidifies into clinkers. After hardening, a set regulating additive, such as gypsum or anhydrite, is combined with the clinkers and this composition is ground into powder, whereby it is ready for present or future use.
2. Description of the Prior Art
Presently, the world is in a situation where energy has become a major consideration and the cost of producing it has steadily increased. Because of the foreseeable shortage of clean burning fuels, such as gas and oil, and due to the increasing cost of such fuels, more industries will have to rely much more heavily on coal as a source of energy. This greater use of coal presents both ecological and technological problems since coal usually burns less cleanly than either gas or oil. Moreover, since coal-driven locomotives and ships have almost disappeared from the scene, coal can scarcely serve at all as a fuel for vehicles. Success in exploiting the world's huge reserves of coal therefore depends on the development of a technology that will convert coal into oil and gas on a large scale. Such technology is currently available in the way of coal gasifiers, with the four principal methods being: carbonization, direct hydrogenation, extraction process, and Fischer-Tropsch Synthesis. Some of the more common types of gasifier processes in use today are: the Koppers-Totsek process, the Hygas process, the Hydrane process, the CO.sub.2 Acceptor process, the Cogas process, the Coed process, the Bi-Gas process and the Atgas process. All of the above processes are described in detail in, Synthetic Fuels Data Handbook, by Cameron Engineers, Inc., Denver, Colo., copyrighted 1975, pages 177-207. This material is incorporated by reference and made a part hereof.
Generally speaking, coal gasification involves the reaction of coal, at high temperatures, with a gas containing oxygen and steam to produce a gas substantially comprising CO and H.sub.2, which is suitable for use as a fuel. As a byproduct of gasification, a char or slag component is produced which must be disposed of. This disposal problem is compounded by the fact that the slag has limited value as a structural material and yet a substantial quantity is produced which must somehow be discarded. Depending upon the type and source of coal, the ash or slag content may vary from 5 to 50 percent by weight. This percentage becomes significant for a gasifier which handles 100,000 lbs./hr. In the past, slag has mainly been disposed of by merely dumping it with little consideration being given to its use for other purposes, mainly because of its limited uses. Accordingly, there is a present need and there will be a greater future need for large volume uses of coal ash slag.
One method for using coal residue has been described by Leon Jules Trief in U.S. Pat. No. 3,759,730 entitled "Process For Utilizing Coal Residues." This patent is directed to a process whereby coal residues, such as power station ash or mining waste products, are mixed with calcium carbonate and fired to about 1300.degree. C. The fired mixture is then heated to at least about 1500.degree. C. to transform it to a molten slag. The molten slag is quenched from about 1500.degree. C. to form granules which are comminuted to get a pure hydraulic binder into which is incorporated a setting and hardening agent. However, the Trief patent does not teach the advantage of this invention, that being the elimination of all the energy required to heat the coal residue to the required temperature in order to form cement. Cement is one of the largest energy consuming products made today and because cement is of such a relatively low value, it is advantageous to conceive of a process which requires less energy. The Trief process demands a large amount of energy because it is utilizing coal residue which is at ambient temperature. Therefore, one using the Trief process would have to fire the slag to 1300.degree. C. before proceeding further and this necessitates a substantial amount of energy.
As used herein, a hydraulic cement refers to a material that will harden in the presence of water and is capable of uniting particles or masses of other solid matter into a concrete mass. In the United States most of the hydraulic cement used in construction is portland cement.
Approximate compositions of hydraulic cements with major components calculated as oxides are shown in Table 1.
TABLE 1 ______________________________________ Type of CaO, SiO.sub.2, Al.sub.2 O.sub.3 Fe.sub.2 O.sub.3 SO.sub.3, MgO, Cement wt. % wt. % wt. % wt. % wt. % wt. % ______________________________________ Expansive 63 19 7 4 4 1 Gypsum plaster 51 2 1 39 1 High-alumina 38 5 38 13 Hydraulic Lime 60 20 8 2 1 Natural 45 25 5 4 2 10 Masonry 50 15 5 2 2 3 Oil Well 63 21 5 6 2 2 Portland 64 21 6 3 3 2 Pozzolanic 45 30 12 4 2 1 Slag 50 26 12 2 2 1 ______________________________________
Portland cement is a finely ground powder, usually gray in color, which when mixed with water, binds together other minerals (sand, gravel, crushed stone) to form concrete, the most widely used construction material. Almost one hundred percent of the cement used in construction today is "portland" or manufactured hydraulic cement--as opposed to "natural" cement widely used a century ago.
Current portland cements are classified into five separate ASTM specifications and these are as follows:
Type I For use in general concrete construction when the special properties specified for Types II, III, IV, and V are not required. PA1 Type II For use in general concrete construction exposed to moderate sulfate action or where a somewhat lower heat of hydration is required. PA1 Type III For use when high early strength is required. PA1 Type IV For use when a low heat of hydration is required. PA1 Type V For use when high sulfate resistance is required.
Portland cement can be manufactured by either the wet or dry process, the dry process being the one adapted to the materials most generally available and commonly used in the United States.
The calcareous material in the dry process is usually limestone and in the wet process marl, being chiefly calcium carbonate (CaCO.sub.3) in either case. The argillaceous material contributing silica (SiO.sub.2) and alumina (Al.sub.2 O.sub.3) for either process can be shale, clay, cement rock (argillaceous limestone), or blast-furnace slag.
In simplified form, the dry process for producing portland cement entails the following operations: (1) preliminary grinding of dry raw materials separately, (2) proportioning, (3) pulverizing the properly proportioned mixture, (4) burning to incipient fusion forming the clinker, (5) cooling and seasoning the clinker, (6) addition of gypsum (calcined or uncalcined) for control of rate of setting, (7) grinding of the clinker to a fine powder that meets the fineness requirements for cement, and (8) storage in bins for seasoning prior to package or bulk shipment. Where the calcareous material occurs as marl, the wet process is commonly employed: (1) the marl is stored in vats as a thin mud or slurry; (2) the clay or other argillaceous material is reduced to a fine powder; (3) the ingredients are proportioned; (4) the ingredients are mixed through a pug mill, after which the burning, cooling, and seasoning, addition of gypsum to control set, final grinding, and storage are carried out as in the dry process.
The greatest energy consumption in the process of making portland cement occurs in the cement kiln where the raw feed must be raised from ambient to fusion temperature, a difference of up to 2800.degree. F. Depending upon the efficiency of any specific cement plant, the energy required to produce a barrel of cement (376 pounds) has been variously estimated at between one half and one million BTU's. This large demand of energy directly affects the cost of the final product. A publication entitled: "Energy Use and Conservation in the U.S. Portland Cement Industry," by Robert D. MacLean, June, 1974, pages 12 and 13, which was presented to the United States Senate Committee on Commerce Public Hearing on Energy Waste in Industrial and Commercial Activities, shows the breakdown of type of fuel consumed and in what stage of the manufacturing process. This data is reproduced in Tables 2 and 3.
The total use of energy by type of fuel in the United States portland cement industry in the year, 1973 is shown in Table 2.
TABLE 2 ______________________________________ Type of Fuel Energy Use ______________________________________ Natural Gas 37% Coal 32% Electric 19% Oil 12% ______________________________________
The relative percentage of total energy required to produce a ton of cement at each step in the manufacturing process is shown in Table 3.
TABLE 3 ______________________________________ Stage of Mfg. Process Energy Consumption ______________________________________ 1. Kiln fuel 86.6% 2. Grinding 7.9% 3. Drying 2.2% 4. Other 2.0% 5. Raw material sources 1.3% ______________________________________
Table 3 demonstrates that the most energy-intensive step in portland cement manufacture is the kiln operation. Although electric power is required for kiln rotation, only fossil fuels are used to generate the 2700.degree. F. temperature required to convert the raw material into clinkers.
It can readily be envisioned that a substantial energy saving can be obtained if molten slag from a coal gasifier could be reacted with a lime containing material to produce portland cement.
It is the general object of this invention to provide a process for converting coal ash slag into a usable product having the characteristics and qualities of portland cement.
It is an object of this invention to provide a process for making portland cement from the slag of a coal gasifier.
It is further an object of this invention to provide a more efficient energy saving way to produce portland cement.
It is still further an object of this invention to provide a useful method for disposing of coal slag.